The invention relates to a process for the biological treatment of liquid wastes using fluidized beds. In particular, it is directed to an apparatus for denitrifying waste water.
Traditionally, sewage treatment plants were designed to remove solids an oxygen-demanding organic material. The plants were not intended to remove algal nutrients such as nitrogen or phosphorus. With the large amounts of fixed nitrogen in the form of ammonia and nitrates that are being introduced into the biosphere by the large scale use of synthetic detergents and fertilizers, and with the demands man makes on his environment owing to population congestion, there definitely appears to be an imbalance developing in our ecological system that may have long range consequences for future generations. Today, municipal wastes generally contain from 25 to 50 milligrams of nitrogen per liter, in the form of ammonia, amines, nitrate, nitrate and the like. The presence of such nutrients in natural waters causes fertilization and vegetative growth in the form of algal blooms. Such blooms often result in accelerated eutrophication.
Traditional sewage treatment processes such as the activated sludge process and trickling filtration can produce effluents with high nitrate concentrations. Further, agricultural run-off contains high concentration of nitrates. Accordingly, there exists an urgent need to reduce the quantity of nitrates and nitrites in waste water prior to returning the water to the natural environment.
Denitrification processes conducted on experimental bases generally involve nitrifying the effluent from contemporary secondary treatment plants to oxidize amines and ammonia to nitrates. The nitrified wastes are then subjected to the action of denitrifying biota which convert the nitrates to nitrites and then to nitrogen gas. The nitrogen gas is then exhausted from the waste water. A carbon source is present during dentrification. As the nitrate nitrogen is reduced to the gaseous nitrogen molecule, a carbon source is oxidized to carbon dioxide and cellular material is also formed.
Traditional denitrification processes require an unusually long detention time, usually ranging from 2 to 4 hours. Such detention times require large and expensive facilities for treatment of industrial or municipal sewage.
Certain experimental denitrification processes have employed downflow columns or beds. Such downflow beds or packed beds tend to be blocked as solids in the waste water are filtered out and further as attached biota undergo uncontrolled growth on the substrate stones or sand. Such blockage causes insurmountable head losses. These losses must be relieved by frequent and impractical back washing.
Generally, upflow expanded beds containing activated carbon have been employed for the removal of small amounts of carbon or biochemical oxygen demand (BOD) that remains after biological treatment or physical/chemical treatment. Biological denitrification has been observed in activated carbon beds operated at a low velocity of approach, approximately 5 gallons per minute per square foot of bed. However, up to now, upflow bacterial denitrification has been regarded as an undesirable phenomenon resulting in formation of uncontrolled biological growth which serves to inhibit or impede the primary function of the bed, the removal of carbon in waste water. Further, only insignificant or inconsequential quantities of nitrogen have been removed from waste water in such processes.
A significant defect in all the prior art experimentation with regard to denitrification of nitrified waste water lies in a failure to remove well over 90% of nitrates, while operating at high flow rates and very low detention times, without plugging.
As employed in the application the term "waste water" or liquid waste includes organic or inorganic liquids or mixtures thereof containing biologically decomposable contaminants and containing the equivalent of at least about 15 milligrams per liter of nitrogen in an oxidized form; particularly the nitrate and/or nitrite form. Municipal and industrial waste waters which have undergone nitrification or contain oxidized nitrogen in the above amounts fall within the above definition of waste water.